add no output to with-system-ffi and with-system-expat

configure.in

@@ -1934,14 +1934,18 @@ AC_PATH_TOOL([PKG_CONFIG], [pkg-config])
 # Check for use of the system expat library
 AC_MSG_CHECKING(for --with-system-expat)
 AC_ARG_WITH(system_expat,
-            AS_HELP_STRING([--with-system-expat], [build pyexpat module using an installed expat library]))
+            AS_HELP_STRING([--with-system-expat], [build pyexpat module using an installed expat library]),
+            [],
+            [with_system_expat="no"])
 
 AC_MSG_RESULT($with_system_expat)
 
 # Check for use of the system libffi library
 AC_MSG_CHECKING(for --with-system-ffi)
 AC_ARG_WITH(system_ffi,
-            AS_HELP_STRING([--with-system-ffi], [build _ctypes module using an installed ffi library]))
+            AS_HELP_STRING([--with-system-ffi], [build _ctypes module using an installed ffi library]),
+            [],
+            [with_system_ffi="no"])
 
 if test "$with_system_ffi" = "yes" && test -n "$PKG_CONFIG"; then
     LIBFFI_INCLUDEDIR="`"$PKG_CONFIG" libffi --cflags-only-I 2>/dev/null | sed -e 's/^-I//;s/ *$//'`"